Paper converting and packaging operations require bonding cellulosic sheet, e.g., paperboard or cardboard, together with adhesives. Typically, waterborne or hot melt adhesives are applied onto one or both substrates and the substrates are compressed together to adhere to one another to form cases, cartons, bag handles, agricultural boxes, bulk box laminations, glued laps, and the like.
Hot melt adhesives are favored over waterborne adhesives in certain areas of the paper converting and packaging operations, where high compression cannot be achieved. Extruded hot melt adhesive lines have high amplitude with a bead-like profile that allows the substrates of the paper products to bond together even with the presence of small gaps. The rapid setting nature of the hot melt adhesives also allows for fast and efficient through-put packaging or converting operations. However, drawbacks to hot melt adhesives include low heat resistance, high energy usage, and non-repulpability.
Waterborne adhesives are typically utilized in paper converting and packaging operations where thin adhesive films rather than thick beads are applied, and where solid and lengthy compression can be achieved; however, the waterborne adhesives do not readily “gap-fill” between two uneven substrates such as those from the undulation of corrugated papers. Although increasing the viscosity, in particular the low shear viscosity, of the waterborne adhesives will help to overcome bead sagging problem and thus achieve “gap-fill,” the viscous adhesives are difficult to extrude due to their high viscosity. Furthermore, water exits (evaporates) from the viscous waterborne adhesive layers or beads ever so slowly that the through-put of such adhesive is much lower than that of hot melt adhesives.
The present invention improves the shortcomings of conventional waterborne adhesives in paper converting and packaging operations through the use of a waterborne adhesive composition with expanded microspheres. The inventive waterborne adhesive may substitute one or more hot melt adhesive bead lines in paper converting and packaging operations as a hot melt assist adhesive for improved performances.